The invention relates to a guide apparatus for the guiding of a workpiece having a porous surface coating in a polishing container, to a method for the polishing of a porous surface layer as well as to the use of the method for the polishing of a porous heat insulating layer of a turbine vane and to a turbine vane polished in accordance with the method of the invention.
A whole series of different processes are known for the polishing of surface layers of workpieces, depending on the application, material and structure of the surface layer. The purpose of the polishing frequently consists of reducing the roughness of the surface in addition to a pure removal of material at the surfaces. This can be desired, for example, for purely aesthetic reasons, for instance to produce glossy surfaces, or it can be required due to technical demands, for example to reduce coefficients of friction, to minimize the adhesion or inclusion of foreign particles such that a required porosity of the surface is maintained, or to prevent soiling of the surface. In the art, as a rule, the roughness of the surface of a solid is characterized by different roughness measuring parameters which can be found in the corresponding technical literature. One of these roughness measuring parameters is the so-called xe2x80x9caverage roughness value Raxe2x80x9d which, as the mean deviation of the absolute amounts of the roughness profile from a central line within a pre-settable measuring path, is a measure for the roughness of a surface and which is given, in dependence on the degree of the roughness, in micrometers (xcexcm) for example.
As already mentioned, different methods are used, depending on the application, for the reduction of the roughness of a surface. For instance, turbine vanes for airplane turbines or for land-based gas turbines for the generation of electrical energy are provided, for example, with layers of metallic alloys, in particular with MCrAlY layers, with M standing for a metal such as nickel (Ni), cobalt (Co) or iron (Fe) and CrAlY (chromium, aluminum, yttrium) designating a super alloy very familiar for this and other purposes. These layers can, for example, be applied in a vacuum chamber in a thickness between 50 xcexcm and 250 xcexcm, with a surface roughness Ra typically being achieved of approximately 6 xcexcm-12 xcexcm. Furthermore, it is frequently necessary to provide the aforesaid MCrAlY layers with a heat insulating layer which the person skilled in the art also frequently calls a TBC coating (thermal barrier coating). Such TBC coatings can be manufactured, for example, on a zirconia (ZrO2) basis, withxe2x80x94in a typical examplexe2x80x94the heat insulating layer being able to be approximately 100 xcexcm up to 500 xcexcm thick, in special cases more than 1 mm and substantially including 92% ZrO2 and 8% yttrium oxide Y2O3 for stabilization. The grain sizes of the grains making up the layer can lie, for example, between 45 xcexcm and 125 xcexcm, with a porosity of the heat insulating layer being typically reached at between 5% and 20%. Typical values for the roughness of TBC coatings are found in the range from 9 xcexcm up to 16 xcexcm. It should be pointed out at this point that the aforesaid parameters of the layers, as well as their chemical composition, can differ considerably from the previously cited examples in a specific case.
The surface roughnesses which the layers show after the application to the workpiece are, however, frequently not acceptable and must be reduced, for example, by polishing.
In the example important for practice of turbine vanes for land-based turbines, values are required for the surface roughness Ra of max. 6 xcexcm, preferably, however, less than 4 xcexcm, in particular in order to prevent increased soiling and to improve flow dynamics.
With MCrAlY layers, or generally with metallic or metal alloy surfaces, the required surface roughness can be achieved using different methods, withxe2x80x94analogous to classical sandblastingxe2x80x94abrasive blasting techniques, for example with fine corundum, shot peening or cut-wire peening with hard steel bodies, with rust-free steel bodies or with ceramic blasting bodies, being customary. To achieve the highest possible surface qualities, i.e. minimum roughness and/or uniform roughness of material surfaces, various methods are available for vibropolishing in combination with polishing elements with an abrasive action.
However, only the last mentioned methods of vibropolishing are used for the polishing of most TBC layers, since they treat the surfaces sufficiently gently in the polishing process such that damage in the form of micro-tears, peeling of surface regions or similar damage in the porous TBC coatings can be avoided.
Two variants of polishing apparatuses are widely used for the carrying out of the vibropolishing, namely so-called round vibrators and tray vibrators. A tray vibrator is an apparatus which substantially includes a polishing container, which includes corresponding polishing elements and which can be set into vibration by suitable devices. The workpieces to be treated are, in the simplest case, placed into the polishing container such that the workpieces are polished by the polishing elements which behave overall under vibration in an analogous manner to a viscid liquid. Partitioning slides can be provided which prevent adjacent workpieces from touching or damaging one another in the polishing container and an external attaching of the workpieces can also be provided. A masking of specific surface regions of the workpiece with covers, in particular made of plastic, can also provide a further protection such that only a partial smoothing of the workpiece is allowed and/or, for example, endangered edges are protected.
These apparatuses known from the prior art have disadvantages which result in unsatisfactory results in particular in the polishing of rotationally asymmetrical workpieces and/or of workpieces having porous surfaces such as turbine vanes with TBC coatings.
For instance, unacceptably high mechanical strains can act on externally clamped workpieces treated in a tray vibrator which, in the worst case, can result in damage to the workpiece and/or to the surfaces to be treated, in particular to porous and/or brittle surfaces. If the workpieces to be polished are placed directly into the polishing container of the tray vibrator in accordance with the prior art, that is, without an external fastening, the risk exists that the workpiece can come into direct contact with the walls of the tray vibrator or with any possibly present partitioning slides and/or with adjacent workpieces, whereby damage to the workpiece or to sensitive regions of the surface of the workpiece, in particular at edges, cannot be precluded. The risk in particular exists that, for example, a distance of less than two polishing elements is adopted between the workpiece surface and an adjacent bounding wall such that a polishing element is jammed between the workpiece surface and an adjacent bounding wall, which can result in enormous point strains on the surface of the workpiece.
Damage of the previously described kind can admittedly be reduced by suitable masking of endangered surface regions. However, this is only possible for those surface regions which do not have to be polished. In addition, this method is very complex in practice since frequently more than one surface region has to be protected separately in each case by a suitable masking, which is associated with a complex installation or removal of the corresponding parts and is thus less efficient from an economic viewpoint. A further substantial disadvantage is the fact that, in particular with rotationally asymmetrical workpieces such as turbine vanes for land-based applications, the known methods result in insufficient surface roughnesses and/or in particular in non-uniformly polished regions, i.e. regions with non-uniform roughness on the surface of the workpiece. Due to the asymmetrical mass distribution, for example of a turbine vane, the turbine vane will only rotate non-uniformly between the polishing elements in the polishing container and the differently oriented surfaces of the turbine vane are acted upon by the polishing elements with different polishing pressures during polishing, which ultimately results in different regions of the surface having different surface roughnesses, and in a sufficiently high roughness not being reached at all at certain surfaces of the turbine vane. What was said above is also true in another respect for polishing methods in which the workpiece is externally fastened. The previously described disadvantages do not only occur on the polishing of turbine vanes, for which the problems are explained here by way of example, but also occur generally in vibropolishing, in particular with rotationally asymmetrical workpieces.
It is therefore an object of the invention to provide a different apparatus and a different method for the polishing of a surface of a workpiece, in particular of a porous surface of a rotationally asymmetrical workpiece.
A guide apparatus made in accordance with the invention for the guiding of a workpiece, in particular of a rotationally asymmetrical workpiece, has a porous surface coating in a polishing container has two guide members spaced apart by at least one spacer member, as well as a holder for the positioning of the workpiece between the two guide members such that the workpiece can be guided in a non-contact manner in the polishing container with respect to the polishing container.
The apparatus in accordance with the invention thus allows a workpiece, in particular a workpiece having a porous surface coating, to be guided, for example, in a tray vibrator during a polishing process, with it being possible to dispense with external fastening means for the guiding of the workpiece. Among other things, external strains can thus be avoided which, in the worst case, can result in damage to the workpiece and/or to the porous surfaces to be treated. It is avoided by the guide apparatus in accordance with the invention that the workpiece to be polished can come into direct contact with any partitioning slides which may be present and/or with adjacent workpieces, whereby damage to the workpiece, or to sensitive regions of the surface of the workpiece, in particular at edges, is largely precluded. Moreover, in particular rotationally asymmetrical workpieces such as turbine vanes can be polished uniformly by use of the guide apparatus in accordance with the invention. This is achieved in that a uniform rotational movement of the workpiece can be achieved on polishing by the design of the guide apparatus even with an asymmetrical mass distribution of the workpiece such as with a turbine vane, such that the workpiece is acted upon by the polishing elements with a substantially uniform polishing pressure, which ultimately results in different regions of the surface having approximately the same surface roughnesses and in a sufficiently low roughness being able to be reached at all surfaces of the turbine vane to be polished.
The guide apparatus includes for this purpose at least two guide members which are spaced apart by at least one spacer member. Furthermore, the guide apparatus has a holder for the positioning of the workpiece between the guide members, with the guide members being arranged and designed such that the workpiece can be guided in a non-contact manner in the polishing container with respect to the bounding walls of a polishing container in which the workpiece is polished, preferably by vibropolishing.
The holder, which is suitable for the positioning of the workpiece in the guide apparatus, can include a cover member in a preferred variant which allows a region of the surface of the workpiece, which should not be polished, to be covered, with the cover member preferably being able to be made of a suitable plastic, but, depending on the requirements, also of other materials such as of a metal or of a ceramic material. It is generally possible for a plurality of cover members to be provided for the covering of different regions of the workpiece which do not have to be part of the holder. The holder and/or the cover member can expediently be made in two or more parts such that the workpiece can be installed in a simple manner in the holder and/or in the cover member. The holder and/or the cover members are preferably fastened to one or more guide members and/or to one or more spacers, preferably by fastening means. The fastening members can, for example, be clamped by elastic plastic bands, by metal bands which are clamped with the aid of suitable devices for the fixing of the workpiece, or can also be realized, for example, by screws or any other suitable fastening means, with the cover member and/or the holder also being able to be designed in one part.
In a particular variant of the guide apparatus in accordance with the operation, it is also possible for a plurality of holders to be provided for the positioning of the workpiece. For instance, the workplace can be suitably positioned between the spacers at the same time, for example, by a plurality of holders of which a respective one or more is/are fixed to a respective guide member and/or to a spacer.
The guide members are preferably designed in ring shape or in the form of a circular disc, with the guide members being arranged parallel to one another by the spacers and being designed such that the guide apparatus is freely rotatable about a longitudinal axis and the workpiece can be guided in the polishing container such that the workpiece does not come into contact with the bounding walls of the polishing container. The guide apparatus is preferably arranged in the polishing container such that the longitudinal axis of the guide apparatus, about which the guide apparatus is freely rotatable, stands substantially perpendicular to the direction of the acting gravity; i.e. the longitudinal axis of the guide apparatus is preferably arranged in the horizontal direction in the polishing container. It is also possible to position the guide apparatus in any other desired orientation in the polishing container.
In the method in accordance with the invention for the polishing of porous surface coatings of a workpiece, in particular of a rotationally asymmetrical workpiece, the workpiece is guided by means of a guide apparatus in a polishing container, with the guide apparatus having two guide members spaced apart by at least one spacer as well as a holder for the positioning of the workpiece between the two guide members. The workpiece is guided in the polishing container in a non-contact manner with respect to the polishing container.
The polishing of a workpiece is preferably carried out in a vibration polishing method, for example by vibropolishing, with the aid of polishing elements.
The polishing container, which contains polishing elements known per se in a known manner, is set into vibration by likewise known devices such that the workpiece fixed in a guide apparatus, which is suitably positioned in the polishing container, is polished by the unordered movement of the polishing elements which behave as a whole in an analogous manner to a viscid liquid. Pyramid-like or tetrahedron-like polishing elements made, for example, of steel, corundum or other suitable materials with typical dimensions in the range of some millimeters can be used as the polishing elements. Polishing elements of a different size or made of different materials can also be used advantageously.
Due to the design of the guide apparatus in accordance with the invention, the guide apparatus preferably also carries out rotational movements about a longitudinal axis in the polishing process in a polishing container with rotationally asymmetrical workpieces such as turbine vanes such that a uniform roughness of the polished surfaces, in particular of the polished porous heat insulating layers (TBC layers), is achieved.
The guide apparatus in accordance with the invention is preferably used for the polishing of turbine vanes having yttrium stabilized heat insulating layers based on ZrO2. Such ceramic heat insulating layers typically include 92% ZrO2, 8% Y2O3, with the grain sizes typically lying between 45 xcexcm and 125 xcexcm in the heat insulating layer with a porosity from 5% to 20%.
In a preferred embodiment of the method in accordance with the invention for the polishing of porous surface layers, a rotationally asymmetrical turbine vane is fixed in a holder which simultaneously acts as a cover member, with the cover member preferably being made in two parts such that the turbine vane can be installed into and removed from the holder or cover member in a simple manner.
The guide apparatus in accordance with the invention, as well as the method in accordance with the invention, are in particular used for the polishing of a porous heat insulating layer (TBC layer) of a turbine vane, with the turbine vane with a porous heat insulating layer polished in accordance with the invention having a surface roughness of less than 15 xcexcm, in particular of less than 8 xcexcm, specifically less than 4 xcexcm.
The invention will be explained in more detail in the following with reference to the drawing.